On 14.8.2013 20:35, Tom Lane wrote: > "Tomas Vondra" <tv@xxxxxxxx> writes: >> I've run into a strange plan difference on 9.1.9 - the first query >> does "DISTINCT" by doing a GROUP BY on the columns (both INT). ... >> Now, this takes ~45 seconds to execute, but after rewriting the >> query to use the regular DISTINCT it suddenly switches to >> HashAggregate with ~1/3 the cost (although it produces the same >> output, AFAIK), and it executes in ~15 seconds. > > [ scratches head... ] I guess you're running into some corner case > where choose_hashed_grouping and choose_hashed_distinct make > different choices. It's going to be tough to debug without a test > case though. I couldn't reproduce the behavior in a few tries here. > >> BTW I can't test this on 9.2 or 9.3 easily, as this is our >> production environment and I can't just export the data. I've tried >> to simulate this but so far no luck. > > I suppose they won't yet you step through those two functions with a > debugger either ... I've managed to get the data to a different machine, and I've spent some time on debugging it. It seems that the difference is in evaluating hashentrysize - while -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
